The present invention relates to a process and an apparatus for producing nonwoven fabric wherein fibrous web is introduced onto a support means and treated with high velocity water streams jetted from above the fibrous web so as to entangle individual fibers in the fibrous web with each other.
Conventional techniques for producing said nonwoven fabric include the following:
1. There have already been proposed a process and apparatus in which the fibrous web is introduced onto a travelling endless mesh screen and treated with high velocity water streams jetted through a plurality of fine orifices from above said fibrous web to achieve fiber entranglement. These are disclosed, for example, by U.S. Pat. No. 3,449,809.
2. Process and apparatus are also well known in which a fibrous web is introduced onto a travelling water impermeable endless belt, treated with high velocity water streams jetted through a plurality of fine orifices from above the fibrous web to achieve preliminary fiber entanglement, then said fibrous web is introduced onto a plurality of water impermeable rollers arranged downstream of said belt at predetermined intervals and on the respective rollers said fibrous web is treated with high velocity water streams jetted from above to achieve multistaged and full fiber entangling effect. These are disclosed, for example, in GB Patent No. 2,085,493B.
3. Process and apparatus have also been known in which the fibrous web is introduced onto support means comprising a combination of a travelling endless mesh screen and a water impermeable member having a narrower supporting surface in contact with the underside of said screen, treated with high velocity water streams jetted through a plurality of fine orifices from above said fibrous web while drainage is effected from the peripheral region of said member under suction so as to achieve a preliminary fiber entangling effect, then said fibrous web is introduced onto a plurality of water impermeable rollers arranged downstream of said screen at a predetermined interval, and, on the respective rollers, said fibrous web is treated again with high velocity water streams jetted through a plurality of fine orifices from above so as to accomplish multistaged and full fiber entangling effect. These are disclosed, for example, in EP Laid-Open Patent Application No. 0,147,904,A2.
According to said technique 1, to produce the nonwoven fabric, the fibrous web is supported on a relatively long continuous mesh screen including an aperture area ratio of 30 to 70% and treated with the water streams jetting on this mesh screen, so that the water streams which have completed their function are smoothly drained through said mesh and said fibrous web is practically free from the draft tending to disturb the fiber orientation. However, the water streams pass too smoothly through said screen to provide rebounding streams generated as a result of striking of the jetted water streams against said screen and fail to promote the desired fiber entanglement. As a consequence, the fiber entangling efficiency is poor and it is impossible to obtain a nonwoven fabric presenting high fiber entangling strength. Furthermore, the individual fibers of said fibrous web tend to twist around yarn crossing points constituting said screen under the action of the jetted water streams, so that some fibers are broken as said fibrous web is peeled off from said screen and remain on said screen, causing a problem of clogging. Such clogging becomes more serious as the water jetting pressure and the water delivery are increased in order to improve the fiber entangling efficiency and the fiber entangling strength. To obtain a nonwoven fabric of a high fiber entangling strength, not only the frequency at which said screen should be exchanged increases but also both said jetting pressure and said water delivery necessarily increase. Additionally, a low productivity is inevitable, resulting in a poor economical efficiency.
From an ideal point of view, said technique 2 is able to improve both the fiber entangling efficiency and the fiber entangling strength with respect to which said technique 1 is disadvantageous, since the jetted water streams do not pass said belt and it is possible for this technique 2 to adequately utilize the energy of the jetted water streams striking against said belt and the rebounding streams thereof for the desired fiber entangling effect. However, from a practical point of view since the water jetting is effected onto the starting fibrous web formed loosely and fluffily on said water impermeable belt, the fibers tend to float in the water streams remaining on said belt, and this results in disturbing the stability of the fiber entangling treatment. To avoid such inconvenience, the jetting pressure of the water streams must be reduced. When the jetting pressure has been thus reduced, the fiber entangling strength is unable to be adequately improved.
Therefore, said fibrous web will be subjected to an excessive draft exerted in the mechanical direction as said fibrous web is transported from one roller to the next roller during the following step and a fiber orientation is given in said direction and a disturbed fiber rearrangement is caused.
Said technique 3 aims to adequately utilize the energy of the jetted water streams striking against said water impermeable member and the rebounding streams thereof. However, another problem encountered by said technique 1, namely, the clogging of said screen can not be eliminated by said technique 3. Furthermore, the stability of fiber entangling treatment (for which said technique 2 is deficient) can be improved by technique 3 to some degree, but not enough to be satisfactory. In consequence, said fibrous web is subjected to an excessive draft exerted in the mechanical direction and given a fiber orientation in this direction as said fibrous web is transported, after being peeled off from said screen, from one roller to the next roller.